1. Field of the Invention
The present invention relates to an organic electroluminescent display device and a method of fabricating an organic electroluminescent display device, and more particularly, to a dual panel-type organic electroluminescent display device and a method of fabricating a dual panel-type organic electroluminescent display device.
2. Discussion of the Related Art
In general, organic electroluminescent display (OELD) devices have an electron-input electrode, which is commonly referred to as a cathode, and hole-input electrode, which is commonly referred to as an anode. The electrons and the holes are supplied to an electroluminescent layer from the cathode and anode, respectively, wherein the electron and hole together form an exciton. The OELD device emits light when the exciton is reduced from an excited state level to a ground state level. Accordingly, since the OELD devices do not require additional light sources, both volume and weight of the OELD devices may be reduced. In addition, the OELD devices are advantageous because of their low power consumption, high luminance, fast response time, and low weight. Presently, the OELD devices are commonly implemented in mobile telecommunication terminals, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, and palm computers. In addition, since manufacturing processes for the OELD devices are simple, manufacturing costs can be reduced as compared to liquid crystal display (LCD) devices.
The OELD devices may be classified into passive matrix-type and active matrix-type. Although the passive matrix-type OELD devices have simple structures and simplified manufacturing processes, they require high power consumption and are not suitable for large-sized display devices. In addition, aperture ratios decrease as the number of electro lines increase. On the other hand, the active matrix-type OELD devices have high light-emitting efficiency and high image display quality.
FIG. 1 is a cross sectional view of an OELD device according to the related art. In FIG. 1, the OELD device 10 has a transparent first substrate 12, a thin film transistor array part 14, a first electrode 16, an organic electroluminescent layer 18, and a second electrode 20, wherein the thin film transistor array part 14 is formed on the transparent first substrate 12. The first electrode 16, organic electroluminescent layer 18, and second electrode 20 are formed over the thin film transistor array part 14. The electroluminescent layer 18 emits red (R), green (G), and blue (B) colored light, and it is commonly formed by patterning organic material separately in each pixel region “P” for the R, G, and B colored light. A second substrate 28 has a moisture absorbent desiccant 22. The OELD device 10 is completed by bonding the first and second substrates 12 and 28 together by disposing a sealant 26 between the first and second substrates 12 and 28. The moisture absorbent desiccant 22 removes moisture and oxygen that may be infiltrated into an interior of the organic ELD 10. The moisture absorbent desiccant 22 is formed by etching away a portion of the second substrate 28, filling the etched portion of the second substrate 28 with moisture absorbent desiccant material, and fixing the moisture absorbent desiccant material with a tape 25.
FIG. 2 is a plan view of a thin film transistor array part of an OELD device according to the related art. In FIG. 2, each of a plurality of pixel regions “P” defined on a substrate 12 includes a switching element “TS,” a driving element “TD,” and a storage capacitor “CST.” The switching element “TS” and the driving element “TD” may be formed with combinations of more than two thin film transistors (TFTs), and the substrate 12 is formed of a transparent material, such as glass and plastic. A gate line 32 is formed along a first direction, and a data line 34 is formed along a second direction perpendicular to the first direction, wherein the data line 34 crosses the gate line perpendicularly with an insulating layer between the gate and data lines 32 and 34. In addition, a power line 35 is formed along the second direction, and is spaced apart from the data line 34.
The TFT used for the switching element “TS” has a switching gate electrode 36, a switching active layer 40, a switching source electrode 46, and a switching drain electrode 50. The TFT for the driving element “TD” has a driving gate electrode 38, a driving active layer 42, a driving source electrode 48, and a driving drain electrode 52. The switching gate electrode 36 is electrically connected to the gate line 32, and the switching source electrode 46 is electrically connected to the data line 34. In addition, the switching drain electrode 50 is electrically connected to the driving gate electrode 38 through a contact hole 54, and the driving source electrode 48 is electrically connected to the power line 35 through a contact hole 56. Further, the driving drain electrode 52 is electrically connected to a first electrode 16 within the pixel region “P,” wherein the power line 35 and a first capacitor electrode 15 that is formed of polycrystalline silicon layer form a storage capacitor “CST”.
FIG. 3 is a cross sectional view along III—III of FIG. 2 according to the related art. In FIG. 3, a first insulating layer (i.e., a buffer layer) 14 is formed on a substrate 12, and a driving element, (i.e., a driving thin film transistor TFT) “TD” including an active layer 42, a gate electrode 38, and source and drain electrodes 48 and 52 is formed on the first insulating layer 14. The active layer 42 is formed on the first insulating layer 14 and a second insulating layer (a gate insulating layer) 37 is interposed between the active layer 42 and the gate electrode 38. In addition, third and fourth insulating layers 39 and 41 are interposed between the gate electrode 38 and the source and drain electrodes 48 and 52. Further, a power line 35 is formed between the third and fourth insulating layers 39 and 41, and connected to the source electrode 48.
A first electrode 16 is formed over the driving TFT “TD” and is connected to the drain electrode 52 of the driving TFT “TD” with a fifth insulating layer 57 between the first electrode 16 and the driving TFT “TD.” An organic electroluminescent (EL) layer 18 is formed on the first electrode 16 for emitting light of a particular color wavelength, and a second electrode 20 is formed on the organic EL layer 18. Accordingly, after forming a sixth insulating layer 58 on the first electrode 16, the sixth insulating layer 58 is patterned to expose the first electrode 16. Then, the organic EL layer 18 and the second electrode 20 are sequentially formed on the exposed first electrode 16, and a storage capacitor “CST” is connected in parallel to the driving TFT “TD,” and includes first and second capacitor electrodes 15 and 35. The source electrode 48 contacts the second capacitor electrode 35 (i.e., a power line), and the first capacitor electrode 15 is formed of polycrystalline silicon material under the second capacitor electrode 35. Moreover, the second electrode 20 is formed on an entire surface of the substrate 12 on which the driving TFT “TD,” the storage capacitor “CST,” and the organic electroluminescent layer 18 are formed.
In the OELD device, a TFT array part and an organic electroluminescent diode are formed over a first substrate, and an additional second substrate is attached with the first substrate for encapsulation. However, when the array part and the organic EL diode are formed on one substrate, a production yield of the organic ELD is determined by a multiplication of TFT's yield and organic emission layer's yield. Since the yield of the organic emission layer is relatively low, the production yield of an ELD is limited by the yield of the organic layer. For example, even when a TFT is properly fabricated, an OELD device using a thin film of about 1000 Å thickness can be determined to be unacceptable due to defects of the organic emission layer. Accordingly, the loss of materials causes an increase in production costs.
In general, OELD device are classified into bottom emission-type and top emission-type according to an emission direction of light used for displaying images. Bottom emission-type OELD devices have the advantages of high encapsulation stability and high process flexibility. However, the bottom emission-type OELD devices are ineffective for high resolution devices because they have poor aperture ratios. In contrast, the top emission-type OELD devices have a higher expected life span because they are easily designed and have a high aperture ratio. However, in the top emission-type OELD devices, the cathode is generally formed on an organic emission layer. As a result, transmittance and optical efficiency of the top emission-type OELD devices are reduced because of a limited number of materials that may be selected. If a thin film-type passivation layer is formed to prevent a reduction of the light transmittance, the thin film-type passivation layer may fail to prevent infiltration of exterior air into the device.